The offset lithographic printing process employs planographic plates which transfer ink to a blanket roll which, in turn, then transfers the ink to a substrate thereby forming the printed images. The plates are referred to as planographic since the image and non-image areas are in the same plane. The plates are construced so that with proper treatment the image areas are hydrophobic and oleophilic and thereby receptive to inks. The non-image areas are hydrophilic and are water receptive. In order to maintain the hydrophilic characteristics on the non-image areas, it is necessary to continuously treat the plate with a water-based fountain solution.
The aqueous fountain solution is used to maintain the non-image areas of a lithographic printing plate insensitive to ink. While an offset printing press is running, fountain solution is continuously applied to the printing plate just before the application of the printing ink, or as a water in ink emulsion. The fountain solution has an affinity for the non-image, hydrophilic areas of the plate and immediately wets these areas. A complete and uniform film of fountain solution prevents the subsequent application of ink from covering the plate in a non-image area. The fountain solution and ink on the plate are then both transferred to the blanket and then to the printing substrate and the process begins again.
Lithographic printing plates are developed to expose a hydrophilic metal surface in the non-image areas, while image areas are left with a hydrophobic surface. There are many fountain solutions which will wet and coat the exposed metal surface of the non-image area of the plate. Plain water may temporarily perform fairly well, although aqueous solutions of various electrolytes, surfactants and water soluble polymers are generally required for good continuous performance. These additives promote plate wetting and fountain solution uniformity, as well as controlling the interaction of the fountain solution with the ink and the substrate.
Acid fountain solutions are the most widely used in commercial printing. While there is a trend toward more use of neutral pH fountain solutions, acidic solutions continue to be widely used because of the proven effectiveness of gum arabic, a water soluble polymer. Gum arabic is a protective colloid that desensitizes the non-image areas of the plate. Since gum arabic is best solubilized and most effective under acidic conditions, acidic fountain solutions continue to be preferred.
Many lithographic presses have a fountain solution distribution system that is separate from the ink distribution system. Generally, the conventional fountain solution distribution system includes a ductor roller which has intermittent or interrupted flow of the fountain solution from the reservoir to dampening form rollers that contact the printing plate. Often these conventional damping systems use paper or molleton (cloth) covered rollers or specially treated rollers in the dampening system roller train to act as intermediate fountain solution reservoirs. Alternatively, brushes can flick droplets of water onto form rollers or directly onto the plate or nozzles can similarly spray a fine-mist.
A growing number of lithographic presses are equipped with a continuous feed dampening system sold by Dahlgren Mfg. Co., Dallas, TX, under the tradename Dahlgren. Other dampening systems of the direct continuous type include the system sold by Miehle-Goss-Dexter, Chicago, IL, under the tradename Miehlematic, and by Harris Corp., Cleveland, OH, under the tradename Duo-Trol and Microflow and by Miller Western Mfg. Co., Pittsburgh, PA, under the tradename Millermatic.
In the Dahlgren system, the printing plate is contacted only by inked rollers, that is, the fountain solution must be carried from the dampening unit rollers by means of one or more inked rollers, usually one of the form rollers, to the printing plate. This type of system requires the assistance of a water tansport additive such as a water soluble glycol as disclosed in U.S. Pat. No. 3,625,715 or an alcohol such as disclosed in U.S. Pat. No. 3,168,037, with isopropanol being almost universally used. The excellent and more independent control of ink and water delivery to the printing plate accounts for the increasing use of this type of dampening system in lithographic printing. This, in turn, accounts for the extensive use of isopropanol in the Dahlgren continuous dampening system. Typically, the fountain solution will contain between about 5 to 30 percent isopropanol depending upon the specific press, speed, type of form and substrate being printed. The use of isopropanol rather than the other alcohols is the best compromise between good press and printing performance and cost of the fountain solution.
Another variety of a continuous contact dampening system is the Millermatic type wherein the fountain solution is applied to the printing plate by means of a dampener form roller that is not part of the inking system. With such an arrangement it would be expected that isopropanol would not be required because the inked form roller is not used to distribute the aqueous fountain solution. Because, however, of the excellent ink and water balance control, it is also common to use isopropanol as a constituent in the dampening solution used with the Millermatic type of dampener.
The typical fountain solution is made up from a fountain solution concentrate, water and an alcohol or alcohol substitute. The fountain solution concentrate generally includes buffering salts, protective colloids, i.e. water-soluble resins or gums such as gum arabic or cellulose gum and frequently a surfactant (wetting agent). The typical fountain solutions for commercial printing are generally acidic and include acidic components such as phosphoric and/or citric acid to maintain a pH value between about 3.5 and 5.5, although neutral and basic fountain solutions are also used.
Alcohol (isopropanol) and alcohol substitutes are commonly added to fountain solutions. These additions are required for efficient operation with certain types of continuous dampening systems (Dahlgren, Duo-Trol, Miehlematic, etc.). Even with conventional systems, smaller amounts of alcohol have frequently been proven to be beneficial. Generally speaking, alcohol will make a borderline dampening solution work better by lowering the surface tension of the water. Also, it minimizes the fountain solution use while maintaining moisture on the plate surface. Reduced water pick up makes it easier for the pressman to maintain the correct ink/water balance. Also, the rapid evaporation of the alcohol from the film of fountain solution on the blanket and printed sheet helps to minimize the paper's tendency to curl. Generally about 5 to 30% of a fountain solution can be isopropanol.
Environmental concerns about press room emissions as well as the cost of alcohol have led to the use of alcohol substitutes. These can perform some, but generally not all, of the functions of isopropanol. Because of these concerns for isopropanol, a number of materials have been suggested as replacements in fountain solutions. Additives such as 2-butoxy ethanol and ethylene glycol have been used as substitutes for isopropanol. U.S. Pat. No. 3,877,372 discloses a fountain solution which includes 2-butoxy ethanol and at least one of hexylene glycol and ethylene glycol, a silicone glycol copolymer and a defoamer type surfactant. U.S. Pat. No. 4,278,467 discloses an isopropanol-free fountain solution which includes an additive having a surface tension less than about 50 dynes/cm such as n-hexoxyethylene glycol (n-hexyl Cellosolve), n-hexoxydiethylene glycol (n-hexyl carbitol), 2-ethyl-1,3-hexanediol, n-butoxyethylene glycolacetate, n-butoxydiethyleneglycolacetate, 3-butoxy-2-propanol and mixtures thereof. U.S. Pat. No. 4,560,410 discloses a fountain solution containing a mixture of a polyol and/or glycol ether partially soluble in water with a polyol and/or glycol ether completely soluble in water.
The use of higher boiling solvents such as glycols, glycol ethers and glycol ether acetates as alcohol substitutes for isopropanol in fountain solutions has resulted in a higher dynamic surface tension. The higher dynamic surface tension reduces the performance and effectiveness of the fountain solution due to decreased wetting action at press speeds. In addition, certain fountain solution concentrate systems containing alcohol substitute systems cannot be supplied in a one-step form because of precipitation of one or more components when mixed with the alcohol substitutes. A one-step fountain solution concentrate is desirable because of the simplicity of metering it on existing dilution equipment.